This invention relates to internally plasticized poly(vinyl chloride) block copolymers. More particularly, this invention relates to thermoplastic block copolymers of the (AB).sub.n type containing alternating rigid poly(vinyl chloride) segments and flexible polyether or polyester segments. The copolymers are useful without the addition of any external plasticizer for applications where externally plasticized PVC has heretofore been used. This invention also relates to processes of forming these block copolymers wherein the rigid and flexible polymer segments are connected with urethane linkages.
Poly(vinyl chloride) (PVC) homopolymer is a rigid thermoplastic material. However, it can be made soft and rubbery by mixing together 30 or more parts of a suitable external plasticizer such as di-2-ethylhexyl phthalate (DOP) per hundred parts of resin. Externally plasticized PVC finds extensive use in automotive materials such as vinyl upholstery fabric and covers for molded instrument panel pads, but plasticizer migration is a problem. Plasticizers tend to evaporate from the PVC at elevated temperatures. Eventually externally plasticized vinyl may lose flexibility and crack due to plasticizer loss.
In our earlier work we discovered that macroazonitrile molecules containing flexible poly(ethylene oxide) segments connected by thermally unstable azo linkages could be heated in the presence of vinyl chloride to initiate addition polymerization of vinyl chloride onto the ends of poly(ethylene oxide) segments. Thus, a flexible segment was incorporated between two rigid PVC segments. A detailed description of the ABA type block copolymer is set out in U.S. Pat. No. 3,949,015. The presence of the flexible polyoxyethylene segment in the block copolymer reduced the need for an external plasticizer. Although this approach mitigated the plasticizer migration problem somewhat, it did not represent a total solution since external plasticizers were still required to form molding compounds with a desired degree of flexibility.